Audion amplifier circuit



June 7, 1932.' 'H. A. SNOW ET AL 1,861,587

AUDION AMPLIFIER CIRCUIT FiledvAug. 13, 1925 INVENTORS:

- fimw M ATTORNEYS.

Patented June 7, 1932 um'rso stars- S PATENT osmos- HABOLD: SNOW AND LEWIS M. HULL, 0F BOONTON, NEW JERSEY, ASSIGNORS, BY

MESNE'ASSIGNMENTS, TORADIO CORPORATION OF AMERICA, OF NEW YORKQN." Y.

A.- CORPORATION OF DELAWARE A'UDION -AMFPLIFIER CIRCUIT.

Application filed August 13, 1925 Serial No; 50,115

' an audion tube. By this compensation We diminish or completely eliminate the efi'ects upon the input circuit terminals due to the variations in potential across the output circuit load In the copending application of Stuart Ballant'ine, Serial No. 34,388, filed June 2, 1925, patented August 13,1929, No. 1,724,819; there is described aWheatstone bridge circuit'whereby such compensation s produced. When this Wheatstone brldgels suhstantiall 'g balancedto eliminate or reduce the How of retroactivecurrents through the grid plate capacity-*ofth'e audion tube, the circuitwhen used as an amplifying stage 1s approximately one-way in action. Moreover this balance is substantially independent of frequency, and the circuit: is admirably zudapted for: use inycascad'e amplification. e have discovered a circuit which whlle retaining all of these advantages, has the additional advantagethat one of the terminals of the input circuit may be connectedi'dlrectly to the audiorr filament. This cannot be done in the construction of an amplifier according to-the Ballantine circuit,although we are aware thatinthe Balla'ntine circuit, aswell asinother wellknow n circuits, an intermediate point of either the inputor the output" circuit may bebrought-to the filament potential; When the input circuit of an ampjlifier stage comprises a tuned circuit containing an inductance and a tuning condenser, it is in many cases'a structural dis-1 advantage to have both sets of-plates of this condenser-permanently above the ground or filament potential, Moreover when the Ballantine. circuit is used in each stage of a cascaded amplifier, it 1 is impossible tobring thetuningcondensers of each stage to acommon terminal, such asground, or the common filament terminal of the several audions. By our invention we have overcome this dificulty and have: succeeded grounding one terminal 'of'th'e; input circuit, or in the case: of a cascaded amplifier having tuned input circuits, bringing all"the' condensers to a common terminal.

Referring HGWQlSO the drawing, Fig. 1 shows a single amplifier stage arranged according to our invention. Fig. 2 is a, diagrammatic representation of the essential alternating current elements of this circuit as they appear when represented in the form of a conventional bridge diagram. F'igi3 is a cascaded'amplifi'er arranged according to our invention.

In Fig. I the inherent grid platecapacity ofthe tube is represented by the dome lines at Cm. The inherent grid filament'capaeit y is similarly shown at Cg. Gonnectedin par allel with (39 there may bean external capacity @Z between the grid and filament; This external capacity hasbeen shownfor the sake of-generalityg but its use may be I unnecessary. The sum of cg; and Gliswhat we term- Go. 'One input termina-l of the" stage is connected directly to the filament while the other input terminal isconnected tothe grid through a coil L2 which is coupled to a coil' L3 inthe" plate'circuit. Coil L2, therefore, is in series with the capacity C0 and the grid and filament of the tube. L2 may be'coupled to a part'or all of'Li-l but in practice a close coupling with the lower portion L5 of the coil L3* is preferred.

.This circuit arrangement will be' seento resulti n a'VVheatstone bridge'in' which one arm is a capacity between grid and filament, another arm isa capacity between grid and plate, a third arm is the portion La of the coil L3, and thefourth arm comp'risesthe portion Lb-of'coil'L3, coil L2, and themutual inductance MQSL The input ofthe audion stage'is between the filament and apoint G which is at the opposite end of'coil 'lfl from that end 2, which is connected to the grid The output of'the stage is between the plate and filament. It is understood that our in-. vention' is not limited t'o'anyparticular form of input andoutput-circuits We have, however, shown as a prefer-red form a tunedinput circuit L102, andv a tuned output circuit L403. i

This arraii'gen'qer'lt of circuit elementsand connections will be more clearin conjunction with Fig. 2 which is a conventional bridge diagram of the essential alternating current elements of the circuit of Fig. 1.

In Fig. 3 two radio frequency stages of amplification I and II, each connected according to our invention, are shown working into a detector stage III.

In all figures such standard elements as filament batteries and rheostats, B. battery by-pass condensers, and the like have been omitted for the sake of simplicity.

By partially or'completely balancing the Wheatstone bridge arrangement herein described a diminution or complete elimination of retroactive effects may be obtained. The conditions for balance of the bridge may be seen from Fig. 2 to be:

and this balance istheoretically independent of frequency. In actual practice we obtain a balance substantially independent of frequency over a considerable range. WVe have found that the various circuit elements can be so designed and proportioned that the effects of distributed capacities in the coils and of other parasitic capacities in a physical structure are negligible over a substantial range of frequencies. a

The potentials of the various, points in an audion circuit are commonly measured with respect to the alternating current potential of the audion filamentpwhich is usually at the lowest potential of the circuit and is commonly grounded. By thelower terminal of either the input or the output circuit we mean that terminal which has the lowest alternating current voltage and hence approaches most nearly to the alternating current potential of the audion filament.

We claim 1. In the process of arranging the impedance elements of anaudion amplifier stage as a Wheatstone bridge network having the output circuit and the input circuit of the stage as the conjugate arms thereof, the step which comprises connecting one terminal of the said input circuit of the stage directly to the filament of the audion.

2. The method of preventing retroaction from the output circuit to the input circuit of an audion amplifier stage having an audion with grid and cathode terminals, the elements of 'said stage being arranged as a Wheatstone bridge capable of being balanced at radio frequencies and including a balancing arm in series with the saidinput circuit between said grid and filament terminals, which. comprises arranging said balancing arm betweenthe grid terminal of the audion and said input circuit.

3. An audion amplifier stage of thetype having a resonant input circuit and animpedance network of the Wheatstone bridge type and capable of being balanced to limit regeneration, said network including as adjacent balancing arms thereof a substantially pure capacity between the grid and filament of the audion, and an impedance between one terminal of the said resonant input circuit and the grid terminal of the audion.

4. An audion amplifier stage wherein an audion and a plurality of impedances form an alternating current Wheatstone bridge network, characterized by the fact that capacity between grid and filament terminals of the audion and capacity between grid and plate terminals constitute adjacent balancing arms of'the' bridge, and a third balancing arm is connected between said grid and the input circuit of the stage.

5. An anti-regenerative audion amplifier stage of the type in which the impedance net work, is arranged in the form of .a Wheatstone bridge circuit having as conjugate arms thereof theinput and the output circuits of said stage, characterized by the fact that one input terminal ofthe amplifier stage is directly connected to the filament of the tube, and that the other input terminal of the stage is connected to the grid of the tube through a coil in inductive relation with the output circuit of the tube. 1

6. A cascaded audion amplifier each stage of which comprises an audion and a plurality of impedances arranged in the form of a Wheatstone bridge circuit having as the conjugate arms thereof the input and the output circuits of said stage and characterized by the fact that one input terminal of the amplifier stage is connected to the filament of the tube, and that the other input terminal of the stage is connected to the grid of the tube through a coil in inductive relation with the output circut of the stage.

7. An audion amplifier stage the impedance net work of which is arranged in the form of a Wheatstone bridge circuit, having as the conjugate arms thereof the input and the output circuits of said stage, v said net work comprising an input circuit having two terminals one of which is'the filament of the audion tube, a capacity between the grid and filament of said tube,and an inductance connected in series between the'grid of said tube and the remaining input terminal, said inductance being coupled to a coil in the plate circuit of said tube.

8. An audion amplifier stage the impedance net work of which is arranged in the form of a Wheatstone bridge having as the conjugate arms thereof the input and the output circuits of said stage, and which comprises an input circuit having a terminal connected to the filament of the audion tube, a capacity between the grid and filament of said. tube, an inductance between the plate and filament of said tube, and a second inductancebetween the grid of said tubeand a second terminal of said input circuit, said second inductance being coupled to said first mentioned inductance.

9. A signal receiving network comprising a plate-filament circuit including an output inductance and a source of direct current energy an input element between the grid and filament, and means for preventing retroaction of the output voltage variations upon the input element including a second inductance inductively coupled to the output inductance, said second inductance being electrically connected between the grid and input element, and an external condenser connected between the grid and one of the other electrodes, the capacity between grid and filament and between grid and plate being so proportioned with respect to the degree of cou pling between the inductances that the network acts as a balanced l/Vheatstone type bridge wherein the terminals of the input element are conjugate with respect to the output circuit.

In testimony whereof, we afiix our signatures.

HAROLD A. SNOW. LEWIS M. HULL. 

